In the past, electrophotographic methods based on application of the Carlson process have been widely used in image formation using toner. Devices adopting the Carlson process are usually provided with a photoreceptive drum, the surface of which is a photoreceptive layer, around which are provided, in order, a charger, an exposure device, a developer, a transfer device, a fixing device, a cleaner, and a charge eliminator.
The Carlson process will be described below.
In this process, first, in a dark environment, the surface of the photoreceptive drum is given a uniform charge by the charger.
Next, the exposure device projects the image of an original onto the surface of the photoreceptive drum, thus eliminating the charge in the areas onto which the light is projected, and forming an electrostatic latent image on the surface of the photoreceptive drum.
Next, toner from the developer, which has a charge of reverse polarity with respect to the photoreceptive drum, is affixed to the electrostatic latent image, thus forming a visible image in toner.
Then, a recording material such as paper is laid over this visible toner image, which is transferred to the recording material by giving the recording paper a charge of reverse polarity with respect to the toner by corona discharge from the reverse side of the recording material.
The toner image is then fixed to the recording material by means of heat and pressure applied by the fixing device, yielding a permanent image.
Toner which remains on the photoreceptive drum without being transferred to the recording material is removed by the cleaner. The electrostatic latent image on the photoreceptive drum is then eliminated by the static eliminator.
Then, successive image formation can be performed by repeating the foregoing process, beginning with charging of the photoreceptive drum.
Toner used in Carlson-process-based electrophotographic methods performs the function of a colored powder to form a visible image, and the functions of carrying a charge and attachment to the recording material. Since toner performs these multiple functions, it is often difficult for a toner to satisfy each of these functions equally well. Sometimes there are problems with image density, at other times problems with preservation, and so on.
In order to solve these problems, additives are often added to the toner to stabilize properties such as preservation, fluidity, and chargeability.
Japanese Examined Patent Publication No. 33698/1988 (Tokukosho 63-33698) discloses a method of manufacturing a developing agent which aims to make effective use of the various properties of toner by achieving the optimum mix of toner and additives.
Here, the additives are in the form of fine particles, but fine particles of this kind are generally found in the form of large secondary particles formed by aggregation of the fine primary particles. For this reason, attempting to provide a toner with desired characteristics usually becomes a question of how finely the aggregates (secondary particles) of additive can be broken down and uniformly dispersed throughout the toner in the optimum state.
Accordingly, the manufacturing method disclosed above adopts as a standard for the optimum state of uniform dispersal of the additives in the toner a mixing time which is 70% of the mixing time at which chargeability of the toner shows a first order singular point. However, depending on the type of additive, there are cases in which the charging level of the additive is lower than that of the toner particles themselves. For this reason, the aggregates of additive cannot be sufficiently broken down by relying solely on the foregoing indicator, and this may make it impossible to obtain desired characteristics.